Since there is only a minuscule (0.6mm) difference in length, it's nearly impossible to measure with a ruler or even to visually see any difference. Instead, look at the side of the cap with the prong. Size 20 extra long caps will have a pinwheel pattern around the prong (as shown in the photo of the snap on the right). The regulars do not. (Note this only applies to glossy snaps.)
Size 20 regular snaps are the most commonly used size. The prong length on these are 5.6mm. They're often found on cloth diapers, cloth menstrual pads, and many household projects that use lightweight material but isn't super thin, stretchy or delicate. (Size 16 snaps are recommended for the latter.)
Size 20 extra long snaps are needed for somewhat thicker material, such as double layers of marine vinyl for key fobs. (If the material is too thick for the snap, the snap will just fall apart.) The prong length of size 20 extra long snaps is 6.2mm.
Tip: if your snaps are not closing properly, most likely the prongs are too long for that particular material. These videos explain why this happens. If you'd rather not buy shorter snaps, just cut the tips off the prongs prior to installation to shorten them.
(Note: If you are working with very thick material, you may need snaps longer than even the size 20 extra longs, such as size 22 or 24.)